1. Field of the Invention
This invention relates to an ophthalmological diagnosis method and an apparatus for carrying out this method, more particularly to an opthalmological diagnosis method and apparatus which use the laser speckle phenomenon for measuring the state of blood flow in the tissue of the eye fundus, the iris and other regions falling within the scope of ophthalmology.
2. Description of the Prior Art
It is known that when a laser beam strikes an object which causes diffusion or scattering of the beam, the light scattered from the body generally gives rise to a speckle pattern caused by interference between reflected rays of the coherent light. In this case, any movement of the body causing the scattering will cause movement of the speckle pattern which can be detected as a time-course change in light intensity at an observation point. Thus, if the changes in intensity are converted into a signal, it becomes possible to measure the movement of the light-scattering body from the signals. The present invention applies this principle to the measurement of the state of blood flow in living tissue such as, for example, the tissue constituting the eye fundus.
One conventional method for measuring blood flow in the eye fundus is the laser-Doppler method described in Applied Optics (Optical Society of America), Vol. 20, No. 1 (January, 1981), pp 117-120. In this method, a laser beam is directed onto the blood vessels of the eye fundus using an eye fundus camera and the frequency shift of the laser light caused by the Doppler effect when the light is scattered by the blood cells flowing through the flood vessels (this shift being proportional to the velocity of the blood cells) is measured and used for calculating the blood flow velocity.
For the detection of the Doppler frequency shift required by this method there are only two possible arrangements. One is split the laser beam into two beams forming equal angles with respect to the optical axis of the original laser beam and to direct the split beams into the eye to be examined such that they intersect precisely at the position of the eye fundus blood vessel concerned. The other is to direct a single laser beam onto the eye fundus blood vessel and to detect the laser light scattered by the blood cells from two different directions. In either case, the required optical system is complex and must be of high precision. Moreover, the fact that the angle of beam incidence or light detection has to be known in advance, the fact that a laser beam adjusted to a beam diameter substantially equal to the diameter of the blood vessel concerned (generally between several tens of .mu.m and several hundred .mu.m) has to be directed onto the blood vessel with high precision, and the fact that the person being subjected to the examination has to be kept stationary during the period of measurement both make this method extremely difficult to apply clinically and greatly impair the repeatability and reliability of the results it produces.
Further, in actual measurements the results are not obtained as a asingle Doppler shift frequency but consist of wide-ranging frequency components extending from the low frequency side to the high frequency side, making it difficult to obtain a reliable absolute velocity value.
Other problems arise from the fact that the laser beam can be directed onto the eye fundus only along paths that are perpendicular or nearly perpendicular to the eye fundus. At such angles, the Doppler shift is very small and the beat signals are hard to detect. This is because the laser Doppler method requires the detection of a single beat component. Thus in applications relating to biological tissues, which produce a wide range of irregular interferences, it is preferable to make use of the laser speckle method, the very essence of which is the interference effect of irregularly scattered light.
As prior art references related to blood flowmeters employing the laser speckle principle there can be mentioned Japanese Unexamined Patent Publication Nos. 60(1985)-199430, 60(1985)-203235 and 60(1985)-203236. The operating principle of the inventions described in these publications is to direct laser light onto the surface of a biological tissue though an optical fiber, to guide the light scattered by the blood cells under the surface of the tissue through another optical fiber to a photomultiplier tube and then either to output a voltage proportional to the frequency of the so-detected speckle signal or to determine the frequency gradient of the speckle signal, in this way measuring the state of blood flow.
While blood flowmeters of this type can be applied to the skin and other biological tissue that enable easy laser light irradiation and detection, they are hard to apply in the field of ophthalmology which deals with a special biological structure that itself constitutes an optical system.
Moreover, for directing the laser light onto the region where the measurement is to be made, these meters require the use of a special probe fitted at the end of the optical fiber. The shape of this probe is such that it prevents accurate observation of the state and location of the region which is actually being illuminated with laser light from the optical fiber within the probe. Further, since the light issuing from the optical fiber spreads with increasing distance from the fiber tip, it is necessary to bring the tip of the fiber near the biological tissue under observation, which is difficult to accomplish in carrying out blood flow measurement in the field of ophthalmology.